Freedom of speech is significantly but not drastically less protected in the UK than in the US.
Please note that I am NOT a lawyer. What follows is merely info I picked up from a long term interest in freedom of speech issues, sharpened by once thinking I was going to be involved in a legal case.
Historically neither English (and I believe also Scottish) statute law nor Common Law gave any specific assertion of the principle to freedom of speech, except (and it is a big "except") in the sense that it was a principle of law that anything not specifically prohibited was permitted. In practice this allowed freedom of speech except where limited by laws against slander, libel, sedition, blasphemy, obscenity, disclosure of official secrets etc.
The situation changed somewhat when the UK acceded to the European Convention of Human Rights in 1951 and more strongly with the Human Rights Act 1998, which incorporated the European Convention on Human Rights into domestic law. Article 10 of the ECHR states that everyone has the right to freedom of expression.
A significant precedent under the ECHR was Handyside vs UK (1976), which specifically stated that ideas that offend, shock or disturb are protected. (Though Handyside still lost his case.)
The Human Rights Act / Article 10 of the European Convention on Human Rights are thus the closest the UK has to an equivalent to the First Amendment of the US Constitution.
The main Act of Parliament curtailing free speech in the UK is the Public Order Act 1986.
Part 3 of the Act makes it a criminal offence to use (Section 18) or publish (Section 19) threatening, abusive or insulting words or behaviour with the intent of stirring up racial hatred where racial hatred is likely to be stirred up.
In 1994 a new section 4A was added regarding harassment "which causes another to fear, on at least two occasions, that violence will be used against him or her; or causes another serious alarm or distress which has a substantial adverse effect on his or her usual day-to-day activities".
In 2006 a new section 3A was added extending it to cover religious hatred.
In 2008 section 3A was extended to cover hatred based on sexual orientation.
See this link to the Crown Prosecution Service for details: http://www.cps.gov.uk/legal/p_to_r/public_order_offences/
Holocaust denial is not illegal in the UK, nor are rude words.
Defamation law is much more harsh in the UK than the US.
Media Law by Robertson and Nicol (5th edition 2008) is a good reference source.